Schottky clamped radio frequency switch

ABSTRACT

Various methods and devices that involve radio frequency (RF) switches with clamped bodies are provided. An exemplary RF switch with a clamped body comprises a channel that separates a source and a drain. The RF switch also comprises a clamp region that spans the channel, extends into the source and drain, and has a lower dopant concentration than both the source and drain. The RF switch also comprises a pair of matching silicide regions formed on either side of the channel and in contact with the clamp region. The clamp region forms a pair of Schottky diode barriers with the pair of matching silicide regions. The RF switch can operate in a plurality of operating modes. The pair of Schottky diode barriers provide a constant sink for accumulated charge in the clamped body that is independent of the operating mode in which the RF switch is operating.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claim the benefit of U.S. patent application Ser. No.14/491,783, filed on Sep. 19, 2014, which is incorporated by referencein its entirety herein for all purposes.

BACKGROUND OF THE INVENTION

Radio frequency (RF) switches are placed in extreme conditions relativeto switches operating in other technical regimes. In certainapplications, these switches need to operate in the hundreds ofgigahertz (GHz) range and handle upwards of one watt of input power in alinear fashion. In addition, RF switches need to exhibit a low on-stateresistance to minimize power consumption and to avoid degrading thesignals on which they operate. To achieve this level of performance, RFswitches often exhibit extremely large widths relative to devices inalternative regimes. For comparison, current digital logic transistorshave widths on the order of nanometers (nm) while the width of an RFswitch transistor can be on the order of millimeters—differing by afactor of more than a million.

Due to the strenuous conditions in which they are required to operate,RF switches face more extreme versions of common problems faced bystandard transistors, and are additionally burdened by a host ofproblems that do not appear in other technical regimes. For example, theaccumulation of charge in the body of a metal-oxide-semiconductor (MOS)RF switch transistor can activate the MOS transistor's parasitic bipolartransistor. In this situation, the accumulated charge in the body of thedevice serves to bias the body of the MOS transistor as if it were thebase of a bipolar transistor. This physical condition results in aperformance degradation effect known as the “kink” effect. It isparticularly problematic in RF switches implemented onsemiconductor-on-insulator (SOI) wafers in which there is no externalbias applied to the body of the transistor because there are no lowimpedance paths for the accumulated charge to follow in order to exitthe body.

In addition to experiencing more problematic versions of common problemsfound in standard transistor technologies, RF switches face additionalperformance degradation from non-ideal physical conditions that are notproblematic in other regimes. Accumulated charge is again also anexample of this kind of physical condition. Given the large width of astandard RF switch, accumulated charge can introduce a nonlineardistortion to the signals on which the RF switch is operating while theRF switch is in the off state. While this may be negligible fortransistors with small widths, the parasitic capacitance caused byaccumulated charge aligned at the body junctions along the entire widthof an RF switch has a significant negative effect on signals coupled tothe terminals of the RF switch when it is in its off state.

RF switch transistors present a particular design challenge as comparedto more standard transistors. Physical effects that cause performancedegradation in standard transistors are felt more acutely in the RFregime. In addition, certain physical effects manifest themselves inperformance degradation modes that are inconsequential outside of the RFregime. Therefore, device engineers working on RF switches employspecialized design methodologies and device configurations to deal withphysical effects that are not generally a concern in other operatingregimes.

SUMMARY OF INVENTION

In one embodiment, a radio frequency (RF) switch comprises a channelthat separates a source and a drain. The RF switch also comprises aclamp region that spans the channel, extends into the source and drain,and has a lower dopant concentration than both the source and drain. TheRF switch also comprises a pair of matching silicide regions formed oneither side of the channel and in contact with the clamp region. Theclamp region forms a pair of Schottky diode barriers with the pair ofmatching silicide regions. The RF switch can operate in a plurality ofoperating modes. The pair of Schottky diode barriers provide a constantsink for accumulated charge in the clamped body that is independent ofthe operating mode in which the RF switch is operating.

In one embodiment, an RF switch comprises a channel separating a sourceand a drain of the RF switch. The RF switch also comprises a first clampregion of a semiconductor material that: (i) spans the channel; (ii)comprises a first contact region extending into the source; and (iii)comprises a second contact region extending into the drain. The RFswitch also comprises a first silicide region formed on the firstcontact region. The RF switch also comprises a second silicide regionformed on the second contact region. The first clamp region of thesemiconductor material has a lower dopant concentration than both thesource and the drain. The first contact region forms a first Schottkydiode barrier with the first silicide region. The second contact regionforms a second Schottky diode barrier with the second silicide region.

In one embodiment, an RF switch comprises a gate that comprises a gateelectrode and a gate insulator. The RF switch also comprises a channelthat: (i) is located in a body of the radio frequency switch; and (ii)is insulated from the gate electrode by the gate insulator. The RFswitch also comprises a first doped region located across the channelfrom a second doped region. The RF switch also comprises a third regionthat: (i) spans the channel; (ii) extends into the first and seconddoped regions; and (iii) has a lower dopant concentration than the firstand second doped regions. The RF switch also comprises a first silicideregion that forms a first Schottky diode junction with the third region.The RF switch also comprises a second silicide region that: (i) islocated across the channel from the first silicide region; and (ii)forms a second Schottky diode with the third.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a plan view of an RF switch portion having a clampregion that is in accordance with embodiments of the present invention.

FIGS. 2A illustrates a cross section of one implementation of the planview in FIG. 1 along reference line A.

FIG. 2B illustrates a cross section of one implementation of the planview in FIG. 1 along reference line B.

FIG. 3 illustrates an energy band diagram of a Schottky barrier that isin accordance with embodiments of the present invention.

FIG. 4 illustrates a plan view of an RF switch portion having two clampregions that are in accordance with embodiments of the presentinvention.

FIG. 5 illustrates a cross section of one implementation of the planview in FIG. 1 along reference line B.

FIG. 6 illustrates a plan view of an RF switch portion having a clampregion that is in accordance with embodiments of the present invention.

FIG. 7 illustrates a cross section of one implementation of the planview in FIG. 6 along reference line E.

FIG. 8 illustrates a flow chart of a method for fabricating an RF switchportion having a clamp region that is in accordance with embodiments ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosedinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe present technology, not as a limitation of the present technology.In fact, it will be apparent to those skilled in the art thatmodifications and variations can be made in the present technologywithout departing from the spirit and scope thereof. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents.

FIG. 1 illustrates a plan view of RF switch portion 100. RF switchportion 100 can be a portion of a single finger of a multiple fingertransistor. Multiple finger transistors are transistors that create aconductive path between two circuit nodes under the control of separatephysical gate electrodes that are commonly coupled to a single gatesignal. The source and drain regions of the multiple fingers can bewholly separate. However, in certain approaches the drain or source oftwo fingers will be a shared physical region that lies between the gateelectrodes of the two fingers. RF switch portion 100 comprises gate 101which is drawn semi-transparent to illustrate portions of a clamp region102 and active region 103 below gate 101. The operation of clamp region102 and active region 103 will be described in more detail below. FIG. 1also includes two reference lines A and B that illustrate therelationship between the plan view of FIG. 1 and the cross sections ofFIGS. 2A and 2B.

A channel separates source 104 from drain 105. The channel is formed inactive region 103 and lies below gate 101. A channel is a region of atransistor that is formed in the body of the transistor and provides aconductive path from the source to the drain in response to a controlsignal on the gate. The active region 103 of RF switch portion 100 canbe implanted with a heavy implant of ions to the source and drainregions using gate 101 as a mask. As a result, the portion of activeregion 103 that lies below gate 101 will serve as a body of the RFswitch and the portions that are not below gate 101 will serve as thesource and drain. An example of how the implant forms the source anddrain as distinct regions from the body is illustrated by cross section200 in FIG. 2A. In certain approaches, the body will also extend belowthe source and drain regions. For example, if the RF switch isimplemented on a bulk substrate, the body will comprise the region ofthe substrate below the source and drain that was not altered by theheavy implant.

Cross section 200 shows one implementation of RF switch portion 100along reference line A. Cross section 200 shows RF switch portion 100implemented in a thin active region 203 which is in accordance with oneimplementation of active region 103. As drawn, the source and drainregions 104 and 105 are implemented as source and drain regions 201 and202 that extend all the way down to buried insulator 204. Buriedinsulator 204 can be the insulator of a semiconductor-on-insulator (SOI)wafer. For example, the insulator could be an oxide. Active region 203can be an ultra-thin silicon layer. The body of RF switch portion 100comprises region 206. The channel of RF switch portion 100 isillustrated in cross section 200 as region 207. Gate 101 includes a gateelectrode and a gate oxide 212 that separates the gate electrode fromthe channel. The width of the channel is measured along a normal vectorto reference line A that lies in the plane of FIG. 1. The length of thechannel is measure along reference line A. As such, reference line A canbe referred to as a lengthwise dimension vector of the channel.

The body of the transistor can be have a dopant concentration in dopantspecies per meter cubed (n/m³) equal to the dopant concentration of theactive region of the original wafer used to form the RF switch. Incertain approaches, this concentration will be in the range of10¹⁴-10¹⁷n/m³. If the RF switch is implemented in an SOI process, theoriginal dopant concentration may be the residual dopant concentrationof the donor wafer used to produce the SOI wafer. In these approaches,the dopant concentration may be on the order of 10¹⁵ n/m³. The body canalso be a well formed in the surface of the wafer that has a differentdopant concentration that the original wafer such as when the RF switchis implemented in a process that provides both n-type and p-typetransistors in a single substrate. The body can also comprise a dopantconcentration set by a V_(th) implant used to alter the thresholdvoltages of active devices formed in the substrate.

A clamp region 102 spans the channel of RF switch portion 100 andextends into source 104 and drain 105. The clamp region has a lowerdopant concentration than the source and the drain. In specificapproaches, the clamp region has the original concentration of theactive layer in which the switch is formed. In some processingapproaches, a clamp region 102 with such characteristics can be formedby creating a hole in the heavy implant used to form the source anddrain using a mask to block the heavy implant. An example of clampregion 102 is illustrated by cross section 210 in FIG. 2B.

Cross section 210 shows one implementation of RF switch portion 100along reference line B. Cross section 210 shows RF switch portion 100implemented in a thin active region 203 that is in accordance with oneimplementation of active region 103. Active region 203, buried insulator204, source region 201, and drain region 202 are the same regions asdescribed in FIG. 2A. In the illustrated example, clamp region 102extends all the way down to buried insulator 204, spans the channel, andextends into source region 201 and drain region 202. The width of clampregion 102 is measured along a normal vector to reference line B thatlies in the plane of FIG. 1. The length of the clamp region is measuredalong reference line B. As such, reference line B can be referred to asa lengthwise dimension vector of the clamp region.

Cross section 210 also illustrates a pair of matching silicide regions211 formed on either side of channel 207. Silicides are commonly used toreduce resistance in integrated circuit processes. As drawn, thesilicide regions 211 cover portions of clamp region 102 as well assource 201 and drain 202. Silicides also form Schottky diode barrierswhen constructed on a semiconductor material, such as silicon, when thesemiconductor material is lightly doped. In this situation, the silicideregions 211 could have been formed in a single step, and could alsocover the surfaces of source 201 and drain 202 in cross section 200 asshown in FIG. 2A.

Clamp region 102 forms a pair of Schottky diode barriers with the pairof matching silicide regions 211. The silicide regions could bespecifically formed over the clamp region and be independently biasedfrom the source and drain contacts. However, as in the illustratedapproach, the silicide regions 211 can be connected to the same contactsas are used to route signals to source 201 and drain 202. This approachprovides a significant benefit in that the silicide step used to treatthe drain and source is also used to create contacts to the drain andsource, and would be conducted regardless of the presence of the clampregion. Therefore, a special processing step is not required to form theSchottky diode junctions. Instead, in these approaches the onlymodification to the processing flow that is required to form the clampregion is a modification to the layout introduced when the source anddrain areas are initially formed.

The clamp region has a lower dopant concentration than the source anddrain. In situations in which the clamp region is formed using a hole inthe implant and the source and drain are formed in a self-alignedmanner, the clamp region could comprise the same material as the body ofthe device such that there was no energy barrier between the clampregion and the body. In situations in which the RF switch wasimplemented in an SOI wafer, this dopant concentration could be theresidual concentration of the donor wafer from which the SOI wafer mayhave been formed. The clamp region could be doped alternatively orcumulatively using a well or threshold voltage implant in situations inwhich the active region was treated with such an implant.

The Schottky diode junctions effectively short out the P-N junctionbetween the heavily implanted region and the body via the clamp region.As the clamp region has a lower dopant concentration that the source anddrain, there is less of an energy barrier between the body and the clampregion than there is between the body and either the source or drain.Due to this low energy barrier, charge carriers in the body that wouldotherwise accumulate in the body can be collected by the clamp regionand syphoned out of the transistor body. The clamp region serves toeffectively clamp the body of the device to a set voltage by providing alow impedance channel for the removal of charge carriers that wouldotherwise build up in the body of the device.

Since the clamp region spans the channel, the relative biasing of thetransistor does not affect the performance of the clampregion—regardless of how the transistor is biased the body never floats.The clamp region provides a constant sink for accumulated charge in theclamped body that is independent of the operating mode of the radiofrequency switch. In the off state, the clamp will remove chargecarriers that could otherwise have produced nonlinear parasiticcapacitance by accumulating in the body at the junction between thesource and body or drain and body. In the on state, the clamp willremove charge that may have otherwise altered the potential of the bodyto a point where the parasitic bipolar junction transistor wasactivated. Therefore, in either operating state, the clamp region willimprove the performance of the device. Furthermore, as the relativebiasing of the device does not matter for the operation of the clampregion, the resulting RF switch can operate as a symmetrical device suchthat the terms source and drain can be used interchangeably to describeregions 201 and 202 based upon how the transistor is biased.

The fact that clamp region 102 spans the channel provides additionalbenefits in terms of the overall process flow. The misalignment ofvarious masks in a semiconductor processing flow can create significantdefects in a semiconductor design. As a basic example, conductive shortscould occur between two portions of the circuit that were meant to beisolated. In the case of a clamp region, it is possible for the clampregion to be aligned outside of the channel region such that it was notin contact with the body and would therefore not serve to removeaccumulated charge from the body of the transistor. However, as clampregion 102 spans the channel, there is almost no chance that anyindividual clamp will wholly fail to serve its purpose. Since anycontact from the body to the drain or source will produce a beneficialresult, the clamp can be misaligned by nearly half of its total lengthand will still function to remove accumulated charge from the body ofthe device. Furthermore, in the case of very short gate lengths, thefact that the clamp region can span the channel provides a significantbenefit in that the alignment tolerance is set by the length of theclamp region and not the length of the channel. This is beneficialbecause the marginal performance effect of increasing the length of theclamp is negligible compared to the effect of increasing the length ofthe transistor channel. Shorter transistor channels produce transistorswith lower on state resistance which, as mentioned above, is a keyperformance metric for RF switches.

FIG. 3 illustrates an energy band diagram 300 of a Schottky diodebarrier formed by the junction of a silicide region and a semiconductorregion. The y-axis of energy diagram 300 is in units of electricalpotential in electron volts and the x-axis is the physical locationalong the junction in meters with the y-intercept being on the silicideside of the junction. As electrons are free in the conductive metal, theenergy diagram on the left side of the junction is illustrated solely bythe Fermi level (E_(F)). The metal side of the energy band diagram canbe fully defined by E_(F) and the work function of the metal—which setsthe minimum energy needed to release a free carrier in the metal. On theright side of the junction, the energy diagram includes the conductionband edge (E_(C)) and the valance band edge (E_(V)) of the semiconductormaterial. The difference between the conduction and valance band edgesdefines the energy band gap of the semiconductor material. Theconduction bands bow upwards towards the junction because zero bias isapplied to the junction, the semiconductor material is n-type, and theE_(F) must be equal on either side of the junction.

The selection of metals and bias concentrations for the semiconductorregion affect the characteristics of the Schottky barrier. In order forthe clamp region to function properly, the clamp should provide aconductive path from the semiconductor region to the metal for excesscharge carriers in the body of the transistor, but should not allowcharge carriers to “leak” by flowing from the metal into thesemiconductor. The zero bias barrier height (φ_(B)) is the differencebetween E_(F) and the conduction band edge at the junction. The biasvoltage required to put the junction in a reverse bias state is set bythe equation: φ_(B)−(E_(C)−E_(F)). The second quantity in this equationis set by the doping level in the semiconductor material and the firstquantity is set predominately by the work function of the metal. Sincethe doping level of the clamp region will often be set to a given levelby other process considerations (e.g., the nominal active doping levelif the clamp is formed through a hole in the implant profile), thebarrier height can be controlled through the selection of a metal havinga particular work function. Various metals can be used to form thesilicide of metal silicide regions 211, such as tungsten, titanium,cobalt, nickel, or, molybdenum. The junction will be less effective atremoving excess charge if a large work function metal is select, but itwill also be less likely to leak.

FIG. 4 illustrates a plan view of RF switch portion 400. RF switchportion 400 can take on any of the characteristics described above withreference to RF switch portion 100. However, RF switch portion 400additionally includes a matching clamp region 401. Matching clamp region401 can take on any of the characteristics described above withreference to clamp region 102. Matching clamp region 401 is parallelwith clamp region 102 and is spaced apart from clamp region 102 alongthe width of gate 101 by a dimension marked using reference line D. Thewidth of both clamp region 102 and matching clamp region 401 are definedalong the width of gate 101 as well. The width of the clamp region isindicated in the plan view of FIG. 4 using reference line C. Theselection of these dimensions is critical for the performance of the RFswitch to which these devices are a part. The particular manner in whichthese dimensions affect the performance of the RF switch will bedescribed in more detail below.

An RF switch can comprise a plurality of clamps spread across aplurality of transistor fingers. For example, RF switch portion 400could be a single finger of a multiple finger transistor that hasadditional clamp regions in other portions of the illustrated transistorfinger as well as on other fingers. The clamps can be evenly spacedalong the fingers of the transistor fingers to assure that the entirebody of the transistor is adequately clamped. Since different transistorfingers can have different orientations or configurations, spacingbetween clamps on a single finger are described as having a certainspacing along a “finger width” of the transistor as opposed to along a“width of the transistor.” When only a single finger is considered,these terms will be equivalent.

The spacing between adjacent clamping regions and the width of eachindividual clamping region are critical dimensions for the performanceof the overall RF switch. Since the on state resistance of an RF switchtransistor is a critical factor, it is particularly important in the RFoperating regime to keep the channel unimpeded along its width. However,the clamp regions act to decrease the effective width of the channelbecause they block the area that would otherwise comprise a forwardbiased diode between the source and body. This concern militates towardskeeping the width of each individual clamp region, as indicated byreference line C in FIG. 4, as small as possible.

Although minimizing the width of each individual clamping region servesto increase the width of an RF switch transistor with all else heldequal, the inventors have discovered an unexpected result when trying tominimize the impact of these clamping regions on the performance of theRF switch. When the width of each clamping region was reduced to below0.2 microns (μm), the clamping regions had no clamping affect. Theclamps were implemented in a silicon SOI wafer having an original dopingconcentration of 10¹⁵ n/m³ and the clamp regions were formed via a holein the source and drain heavy implant. The inventors then determinedthat the silicidation step was causing the small clamp region openingsto close up due to the enhanced diffusion of the dopants in thesilicide. This was an unexpected result that ran counter to the desireto maximize the effective width of the switch. Therefore, in certainapproaches, the width of each clamping region can be kept above 0.2 μm,but should be kept close to 0.2 μm to minimize the on-state resistanceof the RF switch.

Another way in which the effect of the clamping region on the effectivewidth of the RF switch transistor can be mitigated is to increase thespacing between adjacent clamping regions. As shown in FIG. 4, thedimension marked by reference line D could be increased to cover atransistor having a set width while at the same time decreasing theportion of the channel that is blocked by clamping regions. However, thespacing between clamps cannot be increased without causingcountervailing problems. As illustrated in FIG. 4, the farthest anexcess charge carrier 402 will have to travel to reach a clamp region isequal to half of the spacing between adjacent clamp regions. The furtherthe carrier has to travel, the less effective the clamp will be for tworeasons. First, due to the random movement of charge carriers, it willtake longer for the charge carrier to reach the clamp to be removed fromthe system and charge will tend to build up if it is introduced at afaster rate than it is removed. Second, the further the charge carrierhas to migrate through the channel, the larger the IR drop that will becaused by the carriers in aggregate, which will cause the potential ofthe body to rise. In a silicon SOI wafer with an initial dopingconcentration of approximately 10¹⁷ n/m³ to 10¹⁵ n/m³, the spacingbetween adjacent clamps of adequate width should not be increased over25 μm. In most approaches, a spacing of 20 μm should not be exceeded inorder to provide a margin of error and account for the characteristicsof different semiconductor materials and processing technologies.

A final consideration that needs to be taken into account when selectingthe spacing of adjacent clamp regions and the width of each individualclamp is that the total number of clamps required and the width of eachindividual clamp affects the degree of leakage in the RF switch. On asilicon SOI wafer with an initial doping concentration of approximately10¹⁵ n/m³, measurements of roughly 1 nA of leakage were measured at 25°Celsius for a single clamp region with a width of 0.2 μm when thetransistor was biased with 0 v potential difference between the gate andsource, and 3 v potential difference between the drain and source.Although this is a low value relative to the overall current thetransistor passes in the on-state, if clamps are spread throughout theentire length of the transistor, their aggregate leakage can begin toaffect the RF switch performance in a non-negligible fashion. Insituations such as the one described immediately above, the leakage canbe kept to a manageable level by not reducing the spacing between clampsto below 5 μm.

Cross section 500 in FIG. 5 shows another implementation of RF switchportion 100 along reference line B. Cross section 500 shows RF switchportion 100 from FIG. 1 implemented in a thin active region 203 that isin accordance with one implementation of active region 103. Activeregion 203, buried insulator 204, source region 201, and drain region202 are the same regions as described in FIG. 2B. In the illustratedexample, clamp region 102 extends all the way down to buried insulator204, spans the channel and extends into source region 201 and drainregion 202. However, as illustrated, the clamp region is not homogeneousfrom the surface of the substrate down to the buried insulator.

The dopant concentration of clamp region 102 in cross section 500 isstill less than the dopant concentration of source 201 and drain 202.However, clamp region 102 now comprises two regions of moderate doping501. These regions of moderate doping can be formed by a lightly dopeddrain implant or a halo implant. In the approach illustrated by crosssection 500, the clamp region, the source, and the drain are all dopedwith a lightly doped drain implant but the source and drain are alsodoped with a heavy implant. The heavy implant has a heavy implantconcentration. The lower dopant concentration of the clamp region isless than the dopant concentration of the source and drain by thedifference between heavy implant concentration and the moderate dopingconcentration. The benefit of this approach would be that the step usedto conduct the lightly doped drain implant would not need to be modifiedand only the strong implant would need to be altered. Although the clampregion would exhibit a slightly higher barrier to the removal of excesscarriers from the clamped body, in some approaches the cost benefit ofnot having to modify two different implant steps may be beneficial onbalance.

FIG. 6 illustrates a plan view of RF switch portion 600. RF switchportion 600 can take on any of the characteristics described above withreference to RF switch portion 100. However, the clamp regions of RFswitch portion 600 are asymmetric. Clamp regions 601 and 602 are similarto the source side of clamp regions 401 and 102. However, the drain sideof the RF switch comprises a clamp region portion in the entire drainarea 603. The resulting RF switch will not be symmetrical in that theportion of the clamp region 603 that borders drain 105 will sink chargemuch more efficiently and will also leak much more than the portions ofthe clamp region on the source side of the channel. As a result, thedevice should not be used as if it were a symmetrical device. P-typeversions of RF switch portion 600 should be configured so that region105 is biased to a lower potential than region 104 and vice versa forn-type versions of RF switch portion 600. Clamp region portion 603 cantake on any pattern relative to the portions of the clamp region on theother side of the channel. For example, clamp region portion 603 couldinclude twice as many clamp region portions of similar size to the clampregion portions on the alternative side with a clamp region portion onthe drain side interdigitated between each set of adjacent clamps on thesource side. In addition any combination of the variant dopant profilesused to form the clamp region can be used in combination with any of theasymmetrical clamp regions described above. For example, the clampregion could comprise the entire drain of an RF transistor and thesource could be doped with a halo implant while the drain was not dopedwith a halo implant.

FIG. 7 illustrates a cross section 700 of one implementation of RFswitch portion 600 along reference line E. Cross section 700 includes anactive layer 701 that can be implemented on an SOI wafer or a bulksemiconductor wafer. Reference line E is drawn at a point along thewidth of the channel where the clamp region does not span the channel.Instead, source 702 is similar to that of a transistor that does nothave a clamp region while drain 703 includes a Schottky barrier formedbetween silicide 211 and a portion of clamp region 102 that extends intothe drain. As mentioned above, the clamp region 102 has a lower dopantconcentration that source 702 and drain 703. Since low doped drainregion 704 is present throughout the extent of the width of the channel,it is not shorted at any point, and the resulting transistor exhibitsthe benefits of a low doped drain transistor. Therefore, theimplementation of RF switch portion 600 that is illustrated by crosssection 700 has a clamp region to prevent the build-up of unwantedcharge in the body of the device, and also includes a low doped drainregion 704 that is useful for transistors operating a transistor in ahigh power regime.

A method 800 for fabricating an RF switch that is in accordance with thephysical devices described above can be explained with reference to FIG.8. Method 800 begins with step 801 in which a gate is formed over anactive region of a semiconductor substrate. The portion of the activeregion lying immediately underneath the gate can be referred to as thechannel of the RF switch. The gate and channel can take on any of thecharacteristics described above with reference to the previous figures.Additional processing steps that may be conducted along with step 801include the introduction of a well or V_(th) implant to the activeregion of the semiconductor substrate. For example, the optionalintroduction of a well dopant is shown in phantom as step 802.

The method proceeds with step 803 in which a heavy implant is conductedto form the source and drain regions of the RF switch. The source anddrain regions can take on any of the characteristics described abovewith reference to the previous figures. Step 803 can be conducted withthe use of a mask to block the implant from certain portions of thesource and drain region in order to form a clamp region in the activeregion of the device. The resulting clamp region will have a lowerconcentration of dopants that the source and drain region. In certainapproaches, the resulting clamp region will have the same concentrationas the body of the switch transistor. However, the heavy implant of step803 could be followed by a lightly doped drain or halo implant for whichthe mask used to form the clamp region was not used. As a result, theportions of the clamp region that extended into the source and drainwould have a different dopant concentration than the body of the switchtransistor.

The method proceeds with step 804 in which a self-aligned silicide isformed over the clamp regions. As a result, the clamp regions formSchottky diode junctions with the newly created silicide regions. Inspecific approaches, the self-aligned silicide will simultaneously forma silicide over the source and drain regions. In this manner, thecreation of the Schottky diodes does not require an additionalprocessing step, and the only additional layout modification isimplemented using a modified mask during step 703. Given that a mask issometimes required for this step to block remote portions of thesubstrate that will not be exposed to the source drain implant, andgiven the fact that the clamp spans the channel and does not need to bemeticulously aligned, the required modifications to the mask areinexpensive to implement. As a result, the process produces an RF switchwith a clamped body at little to no additional cost as compared to an RFswitch without clamping regions.

Although some embodiments in the above disclosure were specificallyillustrated with reference to RF switches implemented using SOItechnology with silicon as the semiconductor material, alternativetechnologies and materials could be used instead. Exemplary alternativeprocessing technologies include, bulk semiconductor processes,semiconductor-on-oxide, and epitaxial semiconductor processes. Exemplaryalternative semiconductor materials include, silicon, germaniumarsenide, gallium arsenide, gallium nitride, and cadmium telluride.Indeed, any processing technology and semiconductor material thatresults in RF switches that would otherwise suffer performance effectsfrom the presence of excess charge carriers in the body of the switchcould benefit from the teachings herein.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those skilled in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims.

What is claimed is:
 1. A radio frequency switch with a clamped bodycomprising: a channel that separates a source and a drain; a clampregion that spans the channel, extends into the source and drain, andhas a lower dopant concentration than both the source and drain; and apair of matching silicide regions formed on either side of the channeland in contact with the clamp region; a matching clamp region that spansthe channel; wherein the matching clamp region and the clamp region arespaced apart by less than 20 microns; wherein the matching clamp regionand the clamp region are at least 0.2 microns wide along the overallwidth of the channel and wherein the clamp region forms a pair ofSchottky diode barriers with the pair of matching silicide regions. 2.The radio frequency switch of claim 1, wherein: a lengthwise dimensionvector of the matching clamp region is parallel with a lengthwisedimension vector of the clamp region.
 3. The radio frequency switch ofclaim 2, comprising: a plurality of transistor fingers; and a pluralityof additional clamp regions; wherein the plurality of transistor fingerscomprise a plurality of finger widths that each contribute to an overallwidth of the channel; and wherein each additional clamp region in theplurality of additional clamp regions is spaced apart from at least oneother additional clamp region in the plurality of clamp regions by lessthan 20 microns along a finger width in the plurality of finger widths.4. The radio frequency switch of claim 3, wherein: each of the pluralityof additional clamp regions are at least 0.2 microns wide along theoverall width of the channel.
 5. The radio frequency switch of claim 1,wherein: the clamp region comprises silicon; and the source, drain, andclamped body are all in contact with a buried insulator layer of asilicon on insulator wafer.
 6. The radio frequency switch of claim 1,wherein: the clamp region, the source, and the drain are all doped witha lightly doped drain implant that has a moderate implant concentration;the source and drain are also doped with a heavy implant that has aheavy implant concentration; and the lower dopant concentration is lessthan a dopant concentration of the source and drain by at least thedifference between the moderate implant concentration and the heavyimplant concentration.
 7. A radio frequency switch with a clamped bodycomprising: a channel separating a source and a drain of the radiofrequency switch; a first clamp region of a semiconductor material that:(i) spans the channel; (ii) comprises a first contact region extendinginto the source; and (iii) comprises a second contact region extendinginto the drain; a matching clamp region of the semiconductor materialthat spans the channel; a first silicide region formed on the firstcontact region; and a second silicide region formed on the secondcontact region; wherein the first clamp region of the semiconductormaterial has a lower dopant concentration than both the source and thedrain; wherein the first clamp region of the semiconductor material andthe matching clamp region of the semiconductor material are spaced apartby less than 20 microns along a width of the channel; wherein the clampregion and the matching clamp region are at least 0.2 microns wide alongthe width of the channel; wherein the first contact region forms a firstSchottky diode barrier with the first silicide region; and wherein thesecond contact region forms a second Schottky diode barrier with thesecond silicide region.
 8. The radio frequency switch of claim 7,wherein: a lengthwise dimension vector of the matching clamp region ofthe semiconductor material is parallel with a lengthwise dimensionvector of the first clamp region of the semiconductor material.
 9. Theradio frequency switch of claim 8, comprising: a plurality of transistorfingers; and a plurality of additional clamp regions; wherein theplurality of transistor fingers comprise a plurality of finger widthsthat each contribute to the width of the channel; and wherein eachadditional clamp region in the plurality of clamp regions is spacedapart from at least one other additional clamp region in the pluralityof clamp regions by less than 20 microns along a finger width in theplurality of finger widths.
 10. The radio frequency switch of claim 9,wherein: each of the plurality of additional clamp regions are at least0.2 microns wide along the width of the channel.
 11. The radio frequencyswitch of claim 7, wherein: the first clamp region comprises silicon;and the source, the drain, and the clamped body are all in contact witha buried insulator layer of a silicon on insulator wafer.
 12. The radiofrequency switch of claim 7, wherein: the clamp region, the source, andthe drain are all doped with a lightly doped drain implant that has amoderate implant concentration; the source and drain are also doped witha heavy implant that has a heavy implant concentration; and the lowerdopant concentration is less than a dopant concentration of the sourceand drain by at least the difference between the moderate implantconcentration and the heavy implant concentration.
 13. The radiofrequency switch of claim 7, wherein: the first contact region extendsinto the source along a first portion of a width of the channel; thesecond contact region extends into the drain along a second portion ofthe width of the channel; and the first portion of the width is smallerthan the second portion of the width.
 14. The radio frequency switch ofclaim 13, wherein: the second portion of the width of the channel spansa finger width of the channel.
 15. The radio frequency switch of claim14, wherein: the source is doped with a halo implant; and the drain isnot doped with the halo implant.
 16. A radio frequency switchcomprising: a gate that comprises a gate electrode and a gate insulator;a channel that: (i) is located in a body of the radio frequency switch;and (ii) is insulated from the gate electrode by the gate insulator; afirst doped region located across the channel from a second dopedregion; a third region that: (i) spans the channel; (ii) extends intothe first and second doped regions; and (iii) has a lower dopantconcentration than the first and second doped regions; a fourth regionthat: (i) spans the channel; (ii) extends into the first and seconddoped regions; and (iii) has a lower dopant concentration than the firstand second doped regions; a first silicide region that forms a firstSchottky diode junction with the third region; and a second silicideregion that: (i) is located across the channel from the first silicideregion; and (ii) forms a second Schottky diode with the third region;wherein the third region and the fourth region are spaced apart by lessthan 20 microns along a width of the channel; and wherein the thirdregion and the fourth region are each at least 0.2 microns wide alongthe width of the channel
 17. The radio frequency switch of claim 16,wherein: a lengthwise dimension vector of the third region is parallelwith a lengthwise dimension vector of the fourth region.
 18. The radiofrequency switch of claim 17, wherein: the third and fourth regionscomprise a pair of matching clamp regions.
 19. The radio frequencyswitch of claim 18, comprising: a plurality of transistor fingers; and aplurality of additional clamp regions; wherein the plurality oftransistor fingers comprise a plurality of finger widths that eachcontribute to the width of the channel; and wherein each additionalclamp region in the plurality of clamp regions is spaced apart from atleast one other additional clamp region in the plurality of clampregions by less than 20 microns along a finger width in the plurality offinger widths.
 20. The radio frequency switch of claim 19, wherein: thepair of matching clamp regions and the plurality of additional clampregions are each at least 0.2 microns wide along the width of thechannel.
 21. The radio frequency switch of claim 16, wherein: the thirdregion extends into the first doped region along a first portion of awidth of the channel; the third region extends into the second dopedregion along a second portion of the width of the channel; the firstportion of the width is larger than the second portion of the width. 22.The radio frequency switch of claim 16, comprising: the third regioncomprises silicon; and the first doped region, the second doped region,and the body are all in contact with a buried insulator layer of asilicon on insulator wafer.
 23. The radio frequency switch of claim 16,wherein: the third region, first doped region, and the second dopedregion, are all doped with a lightly doped drain implant that has amoderate doping concentration; the first doped region and the seconddope region are also doped with a heavy implant that has a heavy implantconcentration; and the lower dopant concentration is less than thedopant concentration of the first doped region and the second dopedregion by at least a difference between the heavy implant concentrationand the moderate doping concentration.